Why Autumn Begins Tuesday

The height of the noon sun on the winter solstice, as seen from 44 degrees north latitude. The actual height of the sun above your local horizon depends on your latitude, but the total difference between the winter and summer height of the noon sun, for all latitudes, is 47 degrees.Credit: Starry Night Software

The first day of autumn — Sept. 22 this year — is no guarantee of
fall-like weather, but officially the season's start comes around at
the same time each year nonetheless.

Well, sort of.

The first day of autumn arrives on varying dates in different years
for two reasons: Our year is not exactly an even number of days; and
Earth's slightly noncircular orbit, plus the gravitational tug of the
other planets, constantly changes our planet's orientation to the sun
from year to year.

And weather-wise, Earth's seasons have shifted
in the past 150 years or so, according to a study that came out earlier
this year. The hottest and coldest days of the years now are occurring
almost two days earlier.

This year, fall starts Tuesday, because that is when the so-called
autumnal equinox occurs (at 5:18 p.m. EDT). Equinoxes (which mark the
onset of spring and autumn) and solstices (which mark when summer and
winter begin) are points in time and space that mark a transition in
our planet's annual trip around the sun.

At each equinox, the sun crosses the Earth's equator, making night
and day of approximately equal length on most of the planet (from the
Latin, equinox means "equal night"). At the equator, the sun is
directly overhead at noon on either equinox.

How it works

Earth's multiple motions — spinning on its axis and orbiting the sun
— are behind everything from day and night to the changing seasons.

The sun comes up each day because Earth rotates once on its axis every 24 hours or so. Seasons are a result of Earth being tilted 23.5 degrees on its spin axis coupled with the planet's 365-day orbit around the sun.

(At the North Pole,
the sun rises only once a year — at the start of spring. It gets higher
in the sky each day until the summer solstice, then sinks but does not
truly set until late September, at the autumn equinox.)

Imagine
Earth as an apple sitting on one side of a table, with the stem being
the North Pole. Tilt the apple 23.5 degrees so the stem points toward a
candle (the sun) at the center of the table. That's summer for the top
half of the apple.

Keep the stem pointing in the same direction but move the apple to
the other side of the table: Now the stem points away from the candle,
and it's winter on the top half of the fruit. The very top of the
apple, representing the north polar region, is in total darkness 24
hours a day, during that season.

At winter solstice, the sun arcs low across the Northern Hemisphere
sky for those of us below the Arctic Circle, and the stretch of
daylight is at its shortest. By the time of the spring equinox, days
have grown noticeably longer. At the summer solstice, the sun gets as
high in our sky as it can go, yielding the longest day of the year in
the Northern Hemisphere. And another quarter of the way around, at the
autumnal equinox, the planet's axis is again pointing neither toward or
away from the sun.

As long ago as the fourth century B.C., ancient peoples in the
Americas understood enough of this that they could create giant
calendars to interact with the cycle of sunlight. They built observatories of stone
to mark the solstices and other times important for planting or
harvesting crops. Shrines and even tombs were also designed with the
sun in mind.

More seasonal facts

As we orbit the sun, the part of the night sky that's in our view
changes. A given star sets about 4 minutes earlier each night. Over a
month, this amounts to two hours. In winter, this means that we're
looking at stars that during the summer were in our daytime sky,
overwhelmed of course by the glare of the sun. Since we complete a
circle around the sun every year, the stars of summer, such as those in
the Big Dipper, are always the stars of summer.

During summer on the top half of Earth, our planet is actually
farther from the sun than during winter, a fact owing to our
non-circular orbit around the sun. The difference is about 3 million
miles (5 million kilometers), and it makes a difference in radiant heat
received by the entire Earth of nearly 7 percent. But the difference is
more than made up for by the longer days in the Northern Hemisphere
summer with the sun higher in the sky.

Which brings up a common question: If the summer solstice is the
longest day of the year, why are the dog days of August typically
hotter? It takes a while for the oceans to warm up, and a lot of
weather on land is driven by the heat of the oceans.

Robert Roy Britt

Rob was a writer and editor at Space.com starting in 1999. He served as managing editor of Live Science at its launch in 2004. He is now Chief Content Officer overseeing media properties for the sites’ parent company, Purch. Prior to joining the company, Rob was an editor at The Star-Ledger in New Jersey, and in 1998 he was founder and editor of the science news website ExploreZone. He has a journalism degree from Humboldt State University in California.